BacTal is a legendary Runeword created by combining a Bac Rune of Ritual and a Tal Rune of Invocation.
How to Create BacTal
You can create BacTal by adding the legendary Bac Rune of Ritual and the magic Tal Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Bac - Travel 5 meters. (50)
Tal - Invoke the Spiritborn's Pestilent Swarm, dealing damage to enemies. (100)
Overflow
BacTal can not overflow because the amount of Offering you receive from the Bac Rune exactly fills the required Offering of the Tal Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.